Method utilizing formation resistivity measurements for determining formation fluid pressures



1w 4 1 s wl! 8 [am E mn am 3 m m f 3M n 10.0. m M h Tn B E S N Kms T wsE. G C N A SEZ [7.0 H MTW A O O R Em M BHJ E qu G H www. mLEK. T EN? OMEQC.R .J ID 0 V V.. Num :5m RMP OD URN m B 0 Kwn F MF N E CI O O O O MR0. Mmm O m 2 3 4 5 m w w m www i2 Bmw. T152 m12 om; am. n 1 .Frs-v AP Enl LGI T .NN E CII M ZH l0- IR Il. LE l M um l w O., Ungarl .nv .l nu .D9 m m Lw s 7 6T Ru 3 6M 8 l E 9 2 1.. l D; i .mw n n a. 2 if@ .w 0 0 O HLw n o M X f. E.. F e m m.. M m if Ew zma v u i ,l n Q( n D. I u .VviiIlv (CIS (Rc AT 77 F) Ii"J Fell 22, 1966 c. L. BLACKBURN ETAL3,237,094

METHOD UTILIZING FORMATION RESISTIVITY MESUREHENTS DETERMINING FORMATIONFLUID PRESSURBS -Fned sept. 28, 1962 2 sheets-snee: a

SEA WATER soLuTloN TEMPERATURE, F

-b U1 O O DEPTH, FEET I 2 V4 e `|o R OHM-METERS FIG. 5

loo i l l l l lNvENToRs:

0.4 o5 0.6' 0.7 0.a os C' BLACKMN c. E. Ho'rTMAN FLUID PRESSUREGRADIENT. puff' FOR SANDS R K JOHNSON FIG. s BYSWK @2M THEIR ATTORNEY lIIIIIIII- borehole.

' conductor-13 to a generator 14 located at. thesur'face simulate thecommunication between the clay particles while the plates themselvessimulate the clay particles. Upon the application of pressure to theuppermost plate the height of the 4springs between the plates remainsunchanged as long as no water escapes from the system. Thus, in theinitial stage the applied pressure is supported entirely by the equaland opposite pressure of the water. As the water escapes from the systemthrough the perforations in the plate the. uppermost plate will movedownward slightly and the springs will carry part of the applied load.:Asmorewater escapesthe springs will carry additional load until`finally the complete axial load will be borne by the springs` and thesystem will reach a state of equilibrium.

mally high pressures in the fluids contained in permeable rocks whichare enclosed within such shales.

Referring now to FIGURE l, there is shown a simplified diagramk of anelectrical logging tool. The tool consists of two electrodes 10 and 11that are lowered into a The electrode l0 .is coupled by means of a withtl-e generator 14 being coupled to a ground 15. Thus the generator 14will induce a circulating electrical current in the formationsurrounding the borehole 12. TheV electrode 11 is coupled by means of aconductor 16 to a recorder 17 located at the surface.

The recorder 17 measures the potential existing between the electrode 11and ground and this potential will be related to the resistivity of theformation adjacent the electrode 11. The recorder 17 can record theinfomation in various forms with informationl being normally recorded inohm meters. The recorded information will be recorded in relation to thedepth of the electrodes in the wellbore 12. Of course, in actualpractice the electrodes are disposed on an instrument that is loweredinto the wellbore at the end of a logging cable and the informationtransmitted to the surface where it is recorded.

A normal resistivity log that is provided by various logging companiesis referred to as a short normal electrical resistivity log. This log isparticularly adapted for use with this invention since it is notnormally necessary to apply any borehole correction for the conditionsusually encountered in abnormally pressured sections. While this type ofresistivity log is desirable other types of electrical logs may be used,as for example induction, resistivity or normal resistivity. The shaleresistivity (Rsh') as'measured on a short normal electrical log isdependent on four rock variables: porosity (qs), cementation factor (m),temperature (T) and resistivity of the fluid saturating the shaleformation (-Rw). The functional relations between these various factorscan be expressed symbolically by the following formula:

. Rsh=f("1,Rw,,T (I) In the normally pressurized tertiary sections ofthe gulf coast of the United States, three of the variablesporosity,cementation and temperaturecan be expressed as functions of depth, asfollows:

or remains constant as a function of increase indepth is dependent uponthe rate of change of the dependent variable in Equation l above.

Referring now to FIGURE 2, there is shown a plot of the resistivity withrespect to depth for a well drilled that as the well enters the top ofthe abnormal pressure in the gulf coast area. From lthis plot it iseasily seen formation the resistivity decreases rapidly. The data,plotted in FIGURE 2 is uncorrected for the above variables and from thisdata it is seen that the variables do not vary linearly with depth butas some oth-erfunction of depth. v

The well illustrated in FIGUR-E 2 was drilled using normal techniquesandobtaining resistivity logging data at the various intervals. These datawere then plotted as shown in 'FIGURE 2 and a determination made of theexpected rate of increase in resistivity with depth. As the well wasdrilled deeper the rate of increase in the resistivity was noted and thepoint at which thisrate of increase changed or, as shown in FIGURE 2,actually reversed and became a decrease in resistivity was used as thetop of the abnormally pressured section. As shown `in the data in FIGURE2, this occurred at approximately 11,000 feet- At this point, theprocedures required for drilling in abnormally pressured sections werethen instituted and the well deepened to its desired depth.

`From the above it can be seen that the first 11,000 feet of the welllcould be drilled using normal techniques and only the remaining lfewthousand feet would require .the

(RJV- ILA) l-V] X 100 IIn this formula RGN isthe apparent shaleresistivity that is indicated by extrapolation of the resistivity curvefor the'. normally pressured section of the well to the depth ZE, andRGA is the apparent resistivity indicated by the log of the abnormallypressured section at depth 2,. From this curve it is possible todetermine the pressure gradient for any particular position or depth ofwell and thus the net weight required during drilling operations tocontain the pressures that are likely to be encountered. This permitsone to tailor the mud used in drilling to the actual conditions thatexist in the well. This ability to tailor the mud to the requirementsresults in a saving since the use of excessive mud weights can bereduced.

As explained above. the resistivity for the well shown in FIGURES 2 and3 is not a linear relationship with Y depth and thus it is desirable tocorrect a measured resistivity to obtain a linear relationship. It hasbeen discovered that if the measured resistivities are corrected fortemperature alone one will obtain a substantially linear relationshipbetween the logarithm of the measuredl resistivity and depth. A typicalcurve for correcting measured resistivities is shown in FIGURE 4. One ofthe curves shown is for correcting resistivities in shale formations,while the second curve is for correcting resistivities measured insaline solutions. Using this curve to correct the resistivitymeasurements shown in FIGURE 2 and then replotting the infomation, onewould obtain a curve similar to that shown in FIGURE 5. It will be notedfrom the curve shown in FIGURE 5, that the relationship between the.logarithm of shale resistivity versus depth is substantially a linearrelationship. Thus, the. conversion of the resistivity to a linearrelationship to depth provides an easy means for projecting theresistivity in the normal pressured zones to obtain the data requiredfor plotting resistivity parameters against liuid pressure gradients.Using the data from FIGURE 5. one can obtain the curve shown in FIGURE 6which is substantially similar to that shown in FIGURE. 3.

Using the curve plotted in FIGURE 5, one could drill a well in thesamemanner as that described above with relation to FIGURE' 2." Accordingly,one would'drill using normal techniques until a depth lwas reached atwhich the ratelofincrease in resistivity' decreased. This decrease inthe rateof increase in resistivity would then indicate the top of theabnormally pressured section. At this point, of course, one wouldinstitute the techniques required for drilling abnormally pressuredsections.

While this invention has been described with relation to two separatetechniques it should be appreciated that it is susceptible tomanymodifications and changes within its broad spirit and scope. Forexample, while it is preferable to use the amplied short normalelectrical resistivity logs one could also use other types of devicesthat yield formation. resistivity. While this is possible, care must beexercised in using any resistivity log in order to eliminate as manysources of error as possible.

We'claim as our invention: 1. A-methodfor detecting the fluid pressuregradient resistivities in normally pressured shale formations penesuredshale formation comprising:

of a permeable subsurface earth formation penetrated Y' by aborehole,vsaid method comprising: measuringv the individual resistivities of theshale formations penetrated Y, is drilled; and measuring the differencebetween said recorded resistivities in abnormally pressured shaleformations penetrated by the borehole and a projection to the depth ofthe abnormally pressured shale formations of a measured resistivity or'the trend with depth of the measuring the depth at which the boreholeenters an abnormally pressured shale formation;

I measuring the trend with depth of the individual resistivitiesexhibited by shale formations encountered above said abnormallypressured shale formation;

measuring the resistivity of a shale formation at a selected depthencountered below said abnormally pressured shale formation;

determining the difference between the resistivities exhibited at theselected depth by, respectively, a projection to the selected depth ofthe-trend of resistivities exhibited by the shale formations encounteredabove said abnormally pressured shale formation and Y.

the measured resistivity; measuring the amount of ud pressure gradientfrom preplotted values of fluid pressure gradient in relation to thedifference in resistivities that corresponds to the difference betweenthe resistivities exhibited at the selected depth. 3. The methodrofclaim 2 in which the trend with depth of said resistivities is measuredfrom logarithms of resistivities that are corrected for the eEects oftem- WALTER L. CARLSON, Primary Examiner.

1. A METHOD FOR DETECTING THE FLUID PRESSURE GRADIENT OF A PERMEABLESUBSURFACE EARTH FORMATION PENETRATED BY A BOREHOLE, SAID METHODCOMPRISING: MEASURING THE INDIVIDUAL RESISTIVITIES OF THE SHALEFORMATIONS PENETRATED BY THE BOREHOLE; RECORDING THE RESISTIVITIES WITHRELATION TO THE DEPTH OF THE POINT OF MEASUREMENT IN THE BOREHOLE;CONTINUING TO MEASURE SAID RESISTIVITIES AS SAID BOREHOLE IS DRILLED;AND MEASURING THE DIFFERENCE BETWEEN SAID RECORDED RESISTIVITIES INABNORMALLY PRESSURED SHALE FORMATIONS PENETRATED BY THE BOREHOLE AND APROJECTION TO THE DEPTH OF THE ABNORMALLY PRESSURED SHALE FORMATIONS OFA MEASURED RESISTIVITY OF THE TREND WITH DEPTH OF THE RESISTIVITIES INNORMALLY PRESSURED SHALE FORMATIONS PENETRATED BY THE BOREHOLE TODETERMINE THE FLUID PRESSURE GRADIENT OF AN ADJACENT PERMEABLESUBSURFACE EARTH FORMATION.